The invention relates to the field of electro-optical devices and more particularly to microchannel plates (MCPs) and methods for manufacture.
A night vision system converts available low intensity ambient light to a visible image. These systems require some residual light, such as moon or star light as an example, in which to operate. This light is generally rich in infrared radiation, which is invisible to the human eye. The ambient light is intensified by the night vision device to produce an output image which is visible to the human eye. The image intensification process involves conversion of the received ambient light into electronic patterns and the subsequent projection of the electron patterns onto a receptor to produce an image visible to the eye. Typically, the receptor is a phosphor screen which is viewed through a lens provided as an eyepiece.
Specific examples of microchannel plate amplification are found in the image intensifier tubes of the night vision devices commonly used by police departments and by the military for night time surveillance, and for weapon aiming. However, microchannel plates may also be used to produce an intensified electrical signal indicative of the light flux or intensity falling on a photocathode, and even upon particular parts of the photocathode. The resulting electrical signals can be used to drive a video display, for example, or be fed to a computer for processing of the information present in the electrical analog of the image.
In known night vision devices, a photoelectrically responsive photocathode element is used to receive photons from a low light level image. Typically the low light level image is far too dim to view with unaided natural vision, or may only be illuminated by invisible infrared radiation. Radiation at such wavelengths is rich in the nighttime sky. The photocathode produces a pattern of electrons (hereinafter referred to as “photoelectrons”) which correspond with the pattern of photons from the low-level image. Through the use of electrostatic fields, the pattern of photoelectrons emitted from the photocathode is directed to the surface of a microchannel plate.
The pattern of photoelectrons is then introduced into a multitude of small channels (or microchannels) opening onto the surface of the plate which, by the secondary emission of electrons, produce a shower of electrons in a pattern corresponding to the low-level image. That is, the microchannel plate emits from its microchannels a proportional number of secondary emission electrons. These secondary emission electrons form an electron shower thereby amplifying the electrons produced by the photocathode in response to the initial low level image. The shower of electrons, at an intensity much above that produced by the photocathode, is then directed onto a phosphorescent screen. The phosphor of the screen produces an image in visible light which replicates the low-level image.
More particularly, the microchannel plate itself conventionally is formed from a bundle of very small cylindrical tubes, or micro-tubules, which have been fused together into a parallel orientation. The bundle is then sliced to form the microchannel plate. These small cylindrical tubes of the bundle thus have their length arranged generally along the thickness of the microchannel plate. That is, the thickness of the bundle slice or plate is not very great in comparison to its size or lateral extent; however, the microchannels individually are very small so that their length along the thickness of the microchannel plate is still many times their diameter. Thus, a microchannel plate has the appearance of a thin plate with parallel opposite surfaces.
The plate may contain millions of microscopic tubes or channels communicating between the faces of the microchannel plate. Each tube forms a passageway or channel opening at its opposite ends on the opposite faces of the plate. Further, each tube is slightly angulated with respect to a perpendicular from the parallel opposite faces of the plate so that electrons approaching the plate perpendicularly can not simply pass through one of the many microchannels without interacting with the interior surfaces.
Rather than directing the electron shower from a microchannel plate to a phosphorescent screen to produce a visible image, the shower of electrons may be directed upon an anode in order to produce an electrical signal indicative of the light or other radiation flux incident on the photocathode. The electrical analog signal may be employed to produce a mosaic image by electrical manipulation for display on a cathode ray screen, for example. Still alternatively, such a microchannel plate can be used as a “gain block” in a device having a free-space flow of electrons. That is, the microchannel plate provides a spatial output pattern of electrons which replicates an input pattern, and at a considerably higher electron density than the input pattern. Such a device is useful as a particle counter to detect high energy particle interactions which produce electrons.
Regardless of the data output format selected, the sensitivity of the image intensifier or other device utilizing a microchannel plate is directly related to the amount of electron amplification or “gain” imparted by the microchannel plate. That is, as each photoelectron enters a microchannel and strikes the wall, secondary electrons are knocked off or emitted from the area where the photoelectron initially impacted. The physical properties of the walls of the microchannel are such that, generally, a plurality of electrons is emitted each time these walls are contacted by one energetic electron. In other words, the material of the walls has a high coefficient of secondary electron emission or, put yet another way, the electron-emissivity of the walls is greater than one.
Propelled by the electrostatic field across the microchannel plate, the secondary electrons travel toward the far surface of the microchannel plate away from the photocathode and point of entry. Along the way, each of the secondary electrons repeatedly interact with the walls of the microchannel plate resulting in the emission of additional electrons. Statistically, some of the electrons are absorbed into the material of the microchannel plate so that the photoelectrons do not generally escape the plate. However, the secondary electrons continue to increase or cascade along the length of the microchannels. These electrons in turn promote the release of yet additional electrons farther along the microchannel tube. The number of electrons emitted thus increases geometrically along the length of the microchannel to provide a cascade of electrons arising from each one of the original photoelectrons which entered the tube. As discussed above, this electron cascade then exits the individual passageways of the microchannel plate and, under the influence of another electrostatic field, is accelerated toward a corresponding location on a display electrode or phosphor screen. The number of electrons emitted from the microchannel, when averaged with those emitted from the other microchannels, is equivalent to the theoretical amplification or gain of the microchannel plate.
While the intensity of the original image may be amplified several times, various factors can interfere with the efficiency of the process thereby lowering the sensitivity of the device. For example, one inherent problem of microchannel plates is that a photoelectron released from the photocathode may not fall into one of the slightly angulated microchannels, but impacts the bluff conductive face of the plate in a region between the openings of the microchannel tubes. Such bounced photoelectrons, which then produce a number of secondary electrons from a part of the microchannel plate not aligned with the proper location of photocathode generation, decrease the signal-to-noise ratio, visually distorting the image produced by the image intensifier. Other times the errant electron is simply absorbed by a metallized conductive face of the plate and is not amplified to produce part of the image or signal produced by the detector anode.
Of course, one solution to this problem is to increase the amount of microchannel aperture area on the input face of the microchannel plate as was done in U.S. Pat. No. 4,737,013, issued 12 Apr. 1988, to Richard E. Wilcox. Through the use of an etching barrier around each microchannel, these particular microchannel plates have an improved ratio of total end open area of the microchannels to the area of the plate. Specifically, the etching barrier incorporated in the plate allows more precise etching of the microchannel tubes in the plate. The technique allows the plates to be produced with a theoretical open area ratio (OAR) of up to 90% of the plate active surface. As a result, the photoelectrons are not as likely to miss one of the microchannels and impact on the face of the microchannel plate to be bounced into another one of the microchannels. This higher OAR improves the signal-to-noise ratio of image intensification.
A second method to increase the OAR was disclosed in U.S. Pat. No. 5,493,169, issued 20 Feb. 1996, and related division patent U.S. Pat. No. 5,776,538, issued 7 Jul. 1998, to Robert L. Pierle, et al. Pierle taught forming a microchannel plate from first and second cladding glasses and a core glass and then three different etching process steps. Funnel-like openings at each end of the microchannels were disclosed. Multiple etching steps and at least three types of glasses are required to etch away the selected glass portions to form the microchannels. The first cladding glass was used to prevent “the acid from etching completely through the walls of [the] microchannels.” (col. 15, lines 61-63 of U.S. Pat. No. 5,493,169)
While the above cited references introduce and disclose a number of noteworthy advances and technological improvements within the art, none completely fulfills the specific objectives achieved by this invention.